At the moment, optical discs having a recording density of about 50 GB can be commercialized only for domestic civil use by a blue-ray disc (BD) standard or a high definition digital versatile disc (HD DVD) standard using a blue-violet semiconductor layer.
In the future, a large capacity of the same degree as an HDD (hard disc drive) capacity such as 100 GB to 1 TB is put in practical use even in an optical disc.
However, in order to realize such an ultrahigh density in the optical disc, a novel storage technique is required which is different from the trend of a conventional high density technique with the provision of shorter wavelengths and higher objective lens NA up to now.
There is a hologram recording technique for recording digital information by using holography through the study of a next-generation storage technique.
The hologram recording technique is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-272268 (Patent Literature 1). This publication discloses a so-called angle-multiplexing recording system in which a signal optical beam is focused on an optical information recording medium by a lens while a reference light of a parallel optical beam is applied and interferes with the signal optical beam to conduct a hologram record, and further different page data is displayed in a spatial light modulator while changing an incident angle to an optical recording medium of the reference light to conduct multiple recording. Further, this publication discloses a technique in which a signal light is focused by the lens, and an opening (spatial filter) is arranged in a beam waist of the signal light, as a result of which an interval of adjacent holograms can be shortened, and recording density and capacity are increased as compared with the conventional angle-multiplexing recording system.
Also, the hologram recording technique is disclosed in, for example, WO2004-102542 (Patent Literature 2). This publication discloses an example using a shift multiplex system in which a light from a pixel inside of one spatial optical modulator is set as the signal light, and a light from an orbicular zonal pixel outside thereof is set as the reference light, both of those optical beams are focused on the optical recording medium by the same lens, and the signal light and the reference light are allowed to interfere with each other in the vicinity of a focal plane of the lens to record the hologram.
A method of reproducing the hologram described above is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2009-488727 (Patent Literature 3). This publication discloses the reproducing method in which there is provided a two-dimensional Viterbi decoding circuit that calculates a path metric on the basis of a transition of a trellis state of plural rows including a row to be decoded in a two-dimensional reproduced signal of two-dimensionally modulated page data to conduct Viterbi decoding, and the two-dimensional Viterbi decoding circuit removes the transition of the trellis state on the basis of a bit pattern indicative of the transition of the trellis state of the plural rows including the row to be decoded, and a pattern of the two-dimensional modulation to conduct the Viterbi decoding.